1. Field of the Invention
Embodiments of the present invention relate generally to magnetic resonance imaging, and more particularly to a method and apparatus for producing one-, two- and three-dimensional imaging employing a single-sided magnet and a dedicated radio-frequency coil.
2. Discussion of Related Art
In conventional magnetic resonance imaging (MRI), a scanned sample, which may be a patient or a sample of inanimate material, is placed in a substantially uniform, temporally constant main magnetic field within a defined volume. For purposes of the present description, the volume within which meaningful results will be produced is referred to as the sensitive volume. The magnetic field causes nuclear spins within the sample to effectively line up parallel to the field direction. This orientation is permuted by exciting those nuclei with one or many radio-frequency (RF) pulses. As these excited nuclei realign to the external magnetic field, they emit a radio-frequency signal that is detected by a receiver coil. The frequency of the signal the nuclei emit depends on the composition of the nucleus, its surrounding material and on the strength of the external magnetic field. A map of detected nuclei density within the sensitive volume is generated by using non-uniform magnetic fields.
In “single-sided” or unilateral magnetic resonance (MR), the magnetic field is provided in a region outside the probing head by a permanent magnet, an electromagnet or a superconducting magnet. Relatively recent developments in magnetic resonance apparatus have led to the development of devices, which are suitable to a wide variety of measurement applications.
Particularly interesting examples of portable MR devices are described in G. A. Matzkanin, A Review of Nondestructive Characterization of Composites Using NMR in (Non-destructive Characterization of Materials, Springer, Berlin), pp. 655-669 (1989), and in G. Eidmann et al., The NMR MOUSE, a Mobile Universal Surface Explorer, J. Mag. Reson. Series A 122, pp. 104-109 (1996). In this apparatus, a horseshoe magnet extending horizontally comprises north and south poles separated by a gap extending in a transverse horizontal direction. An RF coil is located in the gap between the magnet poles. The size of the sensitive volume is determined by the duration of radio frequency pulses and the magnetic field distribution. The resonance frequency of the RF circuit is either fixed, or it can be tuned over a narrow frequency range. If the frequency is shifted without proper retuning, the received signal degrades rapidly.
Other examples of field-deployable single-sided MR apparatus are described in R. L. Kleinberg et al., Novel NMR Apparatus for Investigating an External Sample, J. Mag. Reson. 97, pp. 466-485 (1992), and in A. Sezginer, RF Sensor of a Novel NMR Apparatus, J. Electromagnetic Waves and Applications 7, pp. 13-30 (1993). In this apparatus, the magnet blocks are shaped in order to produce relatively homogeneous fields in a region remote to the sensor unit.
Spatial resolution may be achieved over the sensitive volume of single-sided MR probes. Magnetic field gradient coils are used to control the field distribution and render images in the plane parallel to the probe surface. Spatial resolution along the gap of a device described in the two previous paragraphs was presented in P. J. Prado et al., One-dimensional Imaging with a Palm-Size Probe, J. Magn. Reson. 144, pp. 200-206 (2000). This instrument has provided a significant capability in providing spatial resolution using phase encoding MRI techniques over a space within the sensitive volume (U.S. Pat. No. 6,489,767).
The devices described above do not allow for effective depth resolution, due to their inherent magnetic field distribution. A frequency shift in the presence of non-flat surfaces of constant magnetic field is associated with a displacement of the sensitive volume with poor spatial resolution. The resonance frequency for these devices is fixed or tunable over a reduced range. Imaging techniques over a selected slice (constant depth) is described in U.S. Pat. No. 5,959,454.